1. Field of the Invention
The present invention relates to modifying the current fed to an aging OLED sub-pixel in order to maintain constant light emission at a desired gray level.
2. Description of the Related Arts
An OLED display is generally comprised of an array of organic light emitting diodes (OLEDs) that have carbon-based films disposed between two charged electrodes. Generally one electrode is comprised of a transparent conductor, for example, indium tin oxide (ITO). Generally, the organic material films are comprised of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When voltage is applied to the OLED, the injected positive and negative charges recombine in the emissive layer and transduce electrical energy to light energy. Unlike liquid crystal displays (LCDS) that require backlighting, OLED displays are self-emissive devices—they emit light rather than modulate transmitted or reflected light.
An OLED display typically includes a plurality of OLEDs arranged in a matrix form including a plurality of rows and a plurality of columns, with the intersection of each row and each column forming a pixel of the OLED display. An OLED display is generally activated by way of a current driving method that relies on either a passive-matrix (PM) scheme or an active-matrix (AM) scheme.
In a passive matrix OLED display, a matrix of electrically-conducting rows and columns forms a two-dimensional array of picture elements called pixels. Sandwiched between the orthogonal column and row lines are thin films of organic material of the OLEDs that are activated to emit light when current is applied to the designated row and column lines. The brightness of each pixel is proportional to the amount of current applied to the OLED of the pixel. While PMOLEDs are fairly simple structures to design and fabricate, they demand relatively expensive, current-sourced drive electronics to operate effectively and are limited as to the number of lines because only one line can be on at a time and therefore the PMOLED must have instantaneous brightness equal to the desired average brightness times the number of lines. Thus, PMOLED displays are typically limited to under 100 lines. In addition, their power consumption is significantly higher than that required by an active-matrix OLED. PMOLED displays are most practical in alpha-numeric displays rather than higher resolution graphic displays.
An active-matrix OLED (AMOLED) display is comprised of OLED pixels that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that emit light upon electrical activation. In contrast to a PMOLED display, where electricity is distributed row by row, the active-matrix TFT backplane acts as an array of switches coupled with sample and hold circuitry that control and hold the amount of current flowing through each individual OLED pixel during the total frame time. The active matrix TFT array continuously controls the current that flows to the OLEDs in the each of pixels, signaling to each OLED how brightly to illuminate.
FIG. 1 illustrates a conventional active matrix OLED display. While the example of FIG. 1 is illustrated as an OLED display, other emissive-type displays would have structures similar to that illustrated in FIG. 1. Referring to FIG. 1, the OLED display panel includes a plurality of rows Row 1, Row 2, . . . , Row Y and a plurality of columns Col. 1, Col. 2, . . . , Col. X arranged in a matrix. The intersection of each row and each column forms a pixel of the OLED display. The OLED display also includes a Gamma network 104, row drivers 116-1, 116-2, . . . , 116-y, column drivers 114-1, 114-2, . . . , 114-x, and a timing controller 112.
For a color OLED display, each pixel includes 3 sub-pixels that have similar structure but emit different colors (R, G, B). For simplicity of illustration, FIG. 1 illustrates only one sub-pixel (denoted as dashed line boxes in FIG. 1, such as box 120) corresponding to one of the R, G, B colors per pixel at the intersection of each row and each column. However, in real OLED display panels, each pixel includes three identical ones of the sub-pixel structure 120 as illustrated in FIG. 1. As shown in FIG. 1, the active drive circuitry of each sub-pixel 120 includes TFTs T1 and T2 and a storage capacitor Cs for driving the OLED D1 of the sub-pixel 120. In the following explanation of FIG. 1, the type of the TFTs T1 and T2 is a p-channel TFT. However, note that n-channel TFTs may also be utilized in the active matrix.
Image data 110 includes data indicating which sub-pixel 120 of the OLED display should be turned on and the brightness of each sub-pixel. Image data 110 is sent by an image rendering device (e.g., graphics controller (not shown herein)) to the timing controller 112, which coordinates column and row timing. The timing controller 112 sends digital numbers (DN) 101 indicating pixel brightness to the gamma network 104. Row timing data 105 included in image data 110 is coupled to the gate lines 150 of each row through its corresponding row driver 116-1, 116-2, . . . , 116-y. Row drivers 116-1, 116-2, . . . , 116-y drive the gate line 150 so that the gate lines 150 carry a voltage of 25 to 30 volts when active. The gates of TFTs T2 of each sub-pixel in a row are connected to gate line 150 of each row to enable TFTs T2 to operate as switches. The data lines 160 are connected to the sources of TFTs T2 in each column. When the gate line 150 becomes active for a row based on the row timing data 105, all the TFTs T2 in the row are turned on. Timing controller 112 sends column timing data 106 to the column drivers 114-1, 114-2, . . . , 114-x. The Gamma network 104 generates the T1 gate voltages 102 (brightness) to be applied to each TFT T1 in the row when the sub-pixel 120 is turned on, based on digital numbers (DNs) 101 corresponding to each gate voltage 102. Column drivers 114-1, 114-2, . . . , 114-x provides analog voltages 160 to be applied to the gates of TFTs T1, corresponding to the T1 gate voltages 102. The voltages 102 representing pixel brightness values are distributed from the Gamma network 104 to all the column drivers 114-1, 114-2, . . . , 114-x in parallel after the appropriate T1 gate voltages 102 have been sent from gamma network 104 to each column driver 114-1, 114-2, . . . , 114-x under control of the column timing data 106 from timing controller 112. Under control of the timing controller 112, for example, row driver 1 (116-1) is activated and all the voltages 102 placed on the column drivers 114-1, 114-2, . . . , 114-x are downloaded to the TFT T1s in row 1. Timing controller 112 then proceeds to send brightness data for the next row (e.g., row 2) using the row driver 2 (116-2) to column drivers 114-1 through 114-x and activating row 2 and so forth, until all rows have been activated and brightness data for the total frame has been downloaded and all the sub-pixels are turned on to the brightness indicated by the image data 110.
The drain of TFT T2 is connected to the gate of TFT T1 and to one side of storage capacitor Cs. The source of TFT T1 is connected to positive supply voltage VDD. The other side of storage capacitor Cs is also connected, for example, to the positive supply voltage VDD and to the source of TFT T1. Note that the storage capacitor Cs may be tied to any reference electrode in the pixel. The drain of TFT T1 is connected to the anode of OLED D1. The cathode of OLED D1 is connected to negative supply voltage Vss or common Ground. The analog voltages 160 are downloaded to the OLED display a row at a time.
When TFT T2 is turned on, the analog T1 gate voltage 160 is applied to the gate of each TFT T1 of each sub-pixel 120, which is locked by storage capacitor Cs. When the row scan moves to the next row, the gate voltage of TFT T1 is locked for the frame time until the next gate voltage for that sub-pixel is sent by the column drivers 114-1, 114-2, . . . , 114-n. In other words, the continuous current flow to the OLEDs is controlled by the two TFTs T1, T2 of each sub-pixel. TFT T2 is used to start and stop the charging of storage capacitor Cs, which provides a voltage source to the gate of TFT T1 at the level needed to create a constant current to the OLED D1. As a result, the AMOLED display operates at all times (i.e., for the entire frame scan), avoiding the need for the very high instantaneous currents required for passive matrix operation. The TFT T2 samples the data on the data line 160, which is held as charge stored in the storage capacitor Cs. The voltage held on the storage capacitor Cs is applied to the gate of the second TFT T1. In response, TFT T1 drives current through the OLED D1 to a specific brightness depending on the value of the sampled and held data signal as stored in the storage capacitor Cs.
FIG. 2 illustrates a conventional gamma network used with an active matrix OLED display. The gamma network 104 is a circuit that converts the brightness data for a sub-pixel from a digital number (DN) representing the desired gray level (brightness) to an analog voltage, which will produce the right amount of current to drive OLED D1 to emit the desired brightness when the analog voltage 160 is applied to the gate of TFT T1 in the sub-pixel 120 (See FIG. 1). For example, the gamma network 104 in FIG. 2 is a conventional 8 bit gamma network used with DN (8 bits) ranging from 0 to 255. Gamma network 104 includes a counter 202, a decoder 204, a series of resistors (R0, . . . , R30, . . . R191, . . . , R223, . . . , R253, R254) (255 resistors for an 8 bit system) and 256 switches GT0, GT1, . . . , GT255. The gate of each switch GT0, GT1, . . . , GT255 is coupled to the corresponding one of the bits of decoder 204. When the corresponding binary bit at the decoder 204 is “1” the corresponding switch (GT0, GT1, . . . , GT255) is turned on, and when the binary bit at the decoder 204 is “0” the corresponding switch (GT0, GT1, GT255) is turned off. DN 101 can be any value between 0 and 255 for an eight bit system. Counter 202 counts up to the value of DN 101 sent to the Gamma network 104, causing decoder 204 to move its output to the gate of the gamma table switches GT(DN). For example, if a DN of 185 indicating brightness level 185 was sent to counter 202, decoder 204 would move its output to GT185, thereby switching switch GT185 on. Gamma network 104 is essentially a voltage divider with 256 taps corresponding to 256 gray levels (brightnesses). The voltage at tap 185 is controlled by switch GT185, which when turned on delivers to the gate of the TFT T1 in the specified sup-pixel the voltage calculated to produce a gray level brightness corresponding to DN 185.
The voltage 102 output from the gamma network 104 is designed to produce a series of currents from TFT T1 that will produce 256 levels (in an 8 bit display system) of light emission from OLED D1 conforming to the brightness response of the human eye. The human eye is logarithmically sensitive to brightness and thus approximately has a linear response approximate to the square of brightness. That is, for the human eye to experience a doubling of brightness, the light flux has to be increased approximately 4 times. This relationship of eye response to light flux (brightness) is known as the gamma function (γ), which is not exactly 2 but closer to 2.2. In general, gamma gives contrast to the image. If, for example, gamma is reduced to 1 (a linear relationship between eye response and light), the images produced would have very low contrast, and be flat and very uninteresting. If gamma is increased, contrast of the image increases. Note that gamma refers to the relationship between the eye and light—not current or voltages. OLED emission is produced by current flowing through OLED D1 as controlled by TFT T1. Thus, it is the function of the gamma network 104 to produce an appropriate voltage, which will produce appropriate current through OLED D1, which will produce light with the correct (or desired) gamma function. The emission of light from OLED material is linear to the current. That is, in order to double the luminance (expressed as cd/m2—candelas per meter squared), current is doubled.
The brightness values in an image are represented as digital numbers (DNs). For an 8-bit display system, DNs range from 0 to 255. The light values are called gray scale levels and are linear to the human eye. Thus, a doubling of DNs is perceived by the human eye as a doubling of brightness. The gamma relation between DNs and the current of TFT T1 can be determined as follows. FIG. 3A illustrates the gamma curve showing the relationship between the digital number (DN) and the OLED current. Note that gamma curve 300 is not linear but has a curve with a changing slope. The exact shape of the gamma curve 300 is determined by the desired gamma. The gamma curve 300 shown in FIG. 3A is for a gamma of 2.
FIG. 3B is a table showing example resistors, voltages and currents for the gamma network in FIG. 2. Referring to FIGS. 2 and 3B, note that the resistors (R0 through R254) are grouped with roughly 32 resistors per group, except Group 0 that includes no resistor, although all the resistors are not shown in FIG. 2 for simplicity of illustration. Each resistor group (Group 0 through Group 8) is associated with a tap voltage Vtap0 through Vtap7 and Vgamma. The tap voltages, for example, are bounded by a minimum voltage (1.541 volts) and a maximum voltage (Vgamma, 12.000 volts). The tap voltages coupled with the minimum and maximum voltages establish the gamma current curve 300 with the aid of resistors R0 through R254. The tap voltages are voltage sources, and thus the voltage established between each resistor is determined by the current drawn between the tap voltages. The greater the number of tap voltages, the better current conformation is to the gamma curve. In the example of FIG. 3B, nine voltage sources produce the voltages at each resistor (R0 through R254), which in turn use TFT T1 to produce the current that conforms to the gamma curve 300. By adjusting the tap voltages, the gamma current curve 300 will change.
The gate voltage 102 to the TFT T1 is determined by the tap voltages, resistors, and which of the switches GT0, . . . , GT255 is turned on. For example, when DN is 255, counter 202 moves the output of decoder 204 to the gate line for GT255; thereby connecting Vgamma voltage to line 102 which connects to the column driver of the selected sub-pixel. Since the Vgamma voltage is the maximum voltage put out by the Gamma Network 104, the maximum voltage is placed on the gate of T1 in the selected sub-pixel. This maximum voltage causes TFT T1 in the selected sub-pixel to supply the current to OLED D1 for the brightest gray level for the sub-pixel. The voltage value of Vgamma is determined by the design of T1 and the designed top brightness of the sub-pixel. The methods of doing such design work are well known in the display industry. The table in FIG. 3B is an example of design voltages for Vgamma and the taps on the voltage divider. For example, the design voltage for Vgamma from FIG. 3B is 12 V. As a further example, if the sub-pixel is scheduled by the image data to be black (off) then DN 0 is sent to the gamma network 104 causing counter 202 to move the output of decoder 204 to switch GT0 connecting Vtap0 to the output line 102. The voltage value of Vtap0 from the table in FIG. 3B is 1.541 Volts, which when supplied to the gate of T1 through the column driver for the selected sub-pixel causes the current supplied to OLED D1 to be less than the threshold current for OLED D1 and therefore, no light will be emitted from the sub-pixel for the frame. The taps on the gamma network voltage divider 104 will be between Vgamma and Vtap0 (12 Volts and 1.541 Volts, respectively, in the example). As a further example, if DN 227 is sent to gamma network 104, counter 202 will move the output of decoder 204 to the gate line for switch GT227 connecting to the aforesaid voltage divider 104 at a point between Vgamma and Vtap7. The exact voltage connected through switch GT227 to output line 102, and thus, to the gate of TFT T1 in the selected sub-pixel will be determined by the voltage drop from Vgamma to Vtap7, which from the table in FIG. 3B is determined to be 12 Volts−10.729 Volts=1.271 Volts. There are 31 resistors (255−224=31) between Vgamma and Vtap7; therefore, the voltage is dropped in 31 equal decrements from Vgamma to Vtap, because all 31 resistors are of the same value, which from the FIG. 3B is 7843 Ohms each. Each voltage drop, therefore, is 1.271/31=0.041 volts. There are 28 resistors (255−227) between the GT227 tap and the GT255 tap; therefore, the voltage drop is 28×0.041=1.148 Volts. The exact voltage sent to the selected sub-pixel through output line 102 and the column driver to the gate of TFT T1 is 12 volts−1.148 Volts=10.852 Volts, which is the T1 gate voltage designed to supply the required current to OLED D1 to emit brightness corresponding to gray level 227. The other voltages at the various gray levels are calculated in the same manner.
Referring back to FIG. 1, the OLED display 100 requires regulated current in each sub-pixel to produce a desired brightness from the pixel. Ideally, the TFTs T1 in each sub-pixel 120 should be good current sources that deliver the same current for the same gate voltage over the lifetime of the OLED display. Also each current source TFT T1 in the active TFT matrix must deliver the same current for the same data voltage stored in the storage capacitor Cs in order that the display is uniform.
Note that there are two types of thin film semiconductors in popular use in the active matrix display industry: amorphous silicon (a-Si) and poly-silicon (p-Si). Emissive displays, such as the active matrix OLED (AMOLED) displays, require high current and stability not available in the a-Si TFTs and therefore typically use p-Si for the TFTs T1, T2. a-Si is converted to p-Si by laser annealing the a-Si to increase the crystal grain size and thus convert a-Si to p-Si. The larger the crystal grain size, the faster and more stable is the resulting semiconductor material. Unfortunately the grain size produced in the laser anneal step is not uniform due to a temperature spread in the laser beam. Thus, uniform TFTs T1, T2 are very difficult to produce and thus the current supplied by TFTs T1 in conventional OLED displays is often non-uniform, resulting in non-uniform display brightness. Non-uniform TFTs T1 throughout the OLED display causes “Mura” or streaking in the OLED displays made with p-Si TFTs. In other words, TFTs T1 may produce different OLED current due to its non-uniformities from sub-pixel to sub-pixel, even if the same gate voltage is applied to the TFTs T1. Therefore, it is necessary to compensate for non-uniformities in the TFTs T1 by applying corrected (compensated) T1 gate voltages that are different from the intended gate voltage from the graphics board (not shown) to the TFTs T1. This can be done by measuring the gray level (gate voltage) versus current characteristics of the TFTs T1 for each sub-pixel, and using such current measurement data to compensate for the non-uniformities in TFTs T1 when driving the TFTs T1 with the gate voltage 102 through the gamma network 104.
Another problem with AMOLED displays occurs due to aging of the material in the OLEDs. As the OLED sub-pixels age with use, OLEDs become less efficient in converting current to light, i.e., the efficiency of light emission of the OLEDs decreases. Thus, as OLED current to light efficiency of the OLED material decreases with use (age), light emitted from an OLED sub-pixel for a given DN number also decreases, because the gamma network 104 in conventional AMOLED does not compensate for the decreased efficiency of light emission in the aged OLED sub-pixels. As a result, the OLED display emits less light for display than desired in response to a given DN. In addition, since the OLED sub-pixels on various parts of the AMOLED display do not age (are not used) equally in a uniform manner, OLED aging also causes non-uniformity in the OLED display.
Thus, there is a need to solve problems associated with aging of the OLED sub-pixels.